The overall objective of this proposal is to understand the genetic and environmental control of type I fimbriae expression. Type I fimbriae are a virulence factor associated with urinary tract infections in E. coli and are attractive to study because they are produced by the extremely well characterized strain, K-12. By studying K-12, we will utilize the extensive genetic tools available in this strain. Fimbrially-mediated attachment to host cells is considered an important step in infection. Fimbriae enable the bacteria to remain attached to the nutrient-rich host epithelium despite the constant flux of fluids. Nevertheless, fimbriae are also-excellent immunogens, and a significant metabolic drain on the bacterium. Therefore, the ability to control fimbrial expression appropriately is likely a virulence factor in itself. The proposed research will focus on the control of the DNA inversion (switch) associated with phase variation of type 1 fimbrial expression. Inversion of the fim switch requires fimB (ON-to-OFF or OFF-to-ON inversion) or fimE (ON-to-OFF inversion only), genes that likely encode the fim switch recombinases. Using molecular biological approaches, we intend to analyze the influence of genetic and environmental factors on fimB and fimE transcription and translation, and on the fim switch directly. The fim switch is also influenced by the global regulatory proteins H-NS and Lrp (Leucine-responsive regulatory protein), two proteins shown to influence the expression of other virulence determinants. The work suggested in this proposal is likely to provide a model with which to compare control of fim in clinical isolates. Moreover, this study should provide an insight into the genetic and environmental signals used to coordinately control virulence factors.